Pressurized water nuclear reactors generally comprise a vessel containing the reactor core which is immersed in the pressurized water for cooling the reactor. The reactor vessel, of cylindrical overall shape, comprises a closure head of hemispherical shape, which can be fitted onto its upper part.
The curved bottom of the vessel is pierced with openings, at each of which a tubular penetration component is secured by welding.
These vessel bottom penetrations comprise an end projecting under the curved bottom, which is connected to a flexible measurement conduit enabling the bottom of the vessel to be connected to an instrumentation room arranged in the structure of the reactor building. Each of the measurement conduits and the corresponding vessel bottom penetration provide passage for a thimble in which a measuring probe travels, secured to the end of a flexible component of great length, and its entry inside the vessel and the core to perform measurements, for example measurements of neutron flux or of temperature, inside the core, while the reactor is operating.
In order to increase the reliability and the operational safety of nuclear reactors and to prolong their useful life, the users are obliged to carry out increasingly numerous checks on the various constituent components of the nuclear reactor and, if appropriate, repairs of defects which may have been detected.
In particular, it may be necessary to check the state of the penetration components of the vessel bottom (and of the vessel closure head penetrations) to ensure that these components are undamaged after a certain period of operation of the reactor, in particular in the region where these penetration components are welded to the bottom of the vessel.
In the event that defects have been detected in the inner surface of a penetration component, these defects, must be repaired, for example by depositing a layer of a metal such as nickel on the inner surface of the penetration component, in the region exhibiting the defects, or else by carrying out an excavation to a certain depth, by machining with removal of matter, of the region exhibiting defects.
The repair operations on the vessel bottom penetration pipes of a nuclear reactor must be performed during a shutdown of the nuclear reactor. The vessel closure head is removed and the operation is carried out through the inside of the bore of the penetration component and by remote control, since the components are situated at the bottom of the vessel and are highly irradiated.
A process and a device have therfore been employed for carrying out simply, rapidly and under complete control, with an excellent surface quality, the excavation of the surface exhibiting defects, i.e. the removal of the material from the component to a limited and perfectly controlled depth so as to generate a new surface which is free from defects.
To this end, applicants' FR-A-92-09,789 discloses a process for machining the cylindrical inner surface of a tubular component such as an adapter secured to the closure head of the vessel of a pressurized water nuclear reactor, by erosion under the effect of a high velocity liquid jet optionally containing a pulverulent abrasive substance.
This process can be employed in particular for carrying out repairs or preventive treatments on the inner surface of an adapter. It results in an improvement of the surface quality, but this machining process is tricky to use and does not make it easily possible to implement accurate control of the thickness of the material which has been removed.
Electrochemical machining was developed above all in the field of aeronautics in the years 1958 to 1965 in the United States and in Europe, as a result of the use in this field of materials which are difficult to machine by conventional machining processes, such as nickel- or cobalt-based refractory metals or alloys or titanium alloys.
At the present time, this process is employed in aeronautics especially for machining vanes; the motor industry employs it for etching stamping dies and components of various types such as big ends, stub axle bearings and rocker arms, manufactured on a large scale in specific shapes and materials.
The electrolytes most commonly employed for machining the majority of metals and alloys, including refractory steels and alloys based on nickel or cobalt, are aqueous solutions of sodium nitrate or sodium chloride.
Other electrolytes, such as acidic or basic (NaOH, NH.sub.4, NH.sub.4 NO.sub.3) solutions are needed for machining some metals and alloys which cannot be dissolved in sodium nitrate or chloride.
In SU-A-1633308 and SU-A-1255325, it has been proposed to use complex mixtures containing in particular sodium nitrate, lithium chloride and copper sulfate or lithium hydroxide and sodium bichromate.
A large number of machining speed curves and of polarization curves have been plotted and published.
However, electrochemical machining processes employing sodium-based electrolytes are generally detrimental to the metal structure.
In fact, sodium in the form of an alkaline compound is one of the potential main causes of caustic corrosion of the intergranular type in nickel-based alloys.
In addition, these processes are low in efficiency and do not allow a material to be machined to an appreciable depth in a sufficiently short time.